354 Dr. C. Callaway — Parallel Structure in Rocks. 



He sees that it is the key of his position. Eeferring (p. 163) to 

 certain masses of schist in granite, he says : — " Since it is obviously 

 impossible that the inclusions taken in and carried up by rocks 

 erupted through deep fissures should be beds of schist 100 to 200 

 feet long, and a series of such beds separated by the fused rock 

 retaining together their parallel position, we have to admit that these 

 indications of bedding are of unobliterated^ bedding." 



In the year of the publication of these views (1881), I first saw 

 the tremendous Highland earth-thrusts, and every succeeding season 

 has taught me more of the wonderful results of earth-pressures. 

 Other workers have been under instruction in the same school ; so 

 that some things which seemed impossible in 1881 have, in 1887, 

 become not only possible, but credible. In the light of recent dis- 

 covery, I venture to suggest that the parallel structure described by 

 Prof. Dana is susceptible of explanation on the theory of intrusion. 

 Igneous rocks forced upwards by tangential pressure would neces- 

 sarily follow the lines of least resistance. These lines might be 

 planes of cleavage or jointing, or, in the case of some schists, seams 

 of soft mineral matter, such as mica or chlorite. We are here con- 

 cerned, with mica-schists, in which we should naturally expect 

 intrusion to correspond with foliation. From this consideration 

 alone, it would therefore seem that the strips of schist lying between 

 intrusive bands would be approximately parallel. But we have not 

 taken into account the horizontal pressure. This cause would pro- 

 duce parallelism even if it did not previously exist. It would 

 compel flattened fragments, lying pell-mell in a plastic ground-mass, 

 to arrange themselves in planes parallel to each other. The size of 

 the fragments would be quite immaterial. The forces which, in the 

 North of Scotland, sent a mass as large as the County of Eutland 

 sliding up-hill for more than a mile, would take small account of 

 little strips, " 100 to 200 feet long," lying immersed in a granitic 

 magma. 



But I am not indulging in mere speculation. In Donegal, I often 

 saw outlying masses of schist immersed in granite, and in some 

 districts their foliation was regularly parallel to the foliation of the 

 region. In these cases. Prof. Dana's theory was clearly inapplicable ; 

 for the junctions between schist and granite were quite sharp, and 

 the mineral variations sometimes observed at junctions indicated, not 

 a gradation of conditions, but contact alteration. Similar cases were 

 also seen in Connemara. Even in hand-specimens, the parallelism 

 of mineral folia was conspicuous ; but, under the microscope, the 

 boundaries of the schist-fragments were perfectly defined. Con- 

 siderable changes, however, had been produced in the enclosing 

 granite, secondary mica and other minerals having originated at the 

 junctions. 



Many other points of great interest are discussed in Prof. Dana's 

 papers ; but some are not essential to my argument, and others are 

 hardly ripe for adequate debate. 



^ The italics are Prof. Dana's. 



